(a) Technical Field of the Invention
The present invention relates to a torque control circuit of an impact tool, which provides a torque control technique that, at the time when the impact tool is operated to hit and tighten a bolt, realizes constant current control and employs an integrated circuit to effect precise control of output impact torque in response to the variation of temperature and voltage.
(b) Description of the Prior Art
An impact tool is used to effect successive impact for tightening a tightened object. Using a sensor to detect the tightened degree of the tightened object at the moment just after an impact in order to effect control of torque is very difficult. Conventionally, displacement sensors are used for such detection, but the result is imprecise and the construction is complicated. Thus, commercialized products were hardly seen heretofore.
With the advance of the power tools, rechargeable impact tools are available in the market, which are of smaller size, lighter weight and are more suitable for carrying than the conventional impact tools that includes a power cable connectable with electric mains, but still provides sufficient output torque to handle bolt/screw tightening. However, the rechargeable impact tool lacks proper torque control means so that the application thereof cannot be expanded, leading to only small quantity manufacturing. If a torque control function can be added, then the rechargeable impact tool may replace the conventional bulky and heavy drill tools.
An idea solution is to make use of the feature that measurement of reduction of motor rotational speed corresponding to power at the time when a direct-current (DC) motor that is arranged inside a housing of an impact tool and provides the primary power output encounters a resistance in its rotation to drive an impact block at a front tip of the tool to work can be converted into a torque. An electronic circuit is used automatically detect the rotational speed for the conversion calculation of corresponding torque value. When a preset torque level is reached, a control of stopping the rotation is effected. However, this is just an ideal condition and is not effective in precisely controlling torque.
Since, theoretically, if the voltage maintains unchanged, by not taking the influence caused by the operating temperature of an electric motor, when the output of the electric motor is subjected to resistance, the characteristics of reduction of internal equivalent resistance, increase of electrical current, and increase of output torque are shown. Thus, by detecting the rotational speed to correspond to the output torque of the electric motor, a reference for adjusting and controlling the maximum and final rotating and tightening torque can be obtained. However, detecting rotational speed to correspond to the torque of the electric motor is still of various variable factors, and further, resolution and instantaneity of monitoring of the rotational speed of the electric motor by means of electronic circuits are insufficient. In practical applications, the voltage level of the rechargeable batteries reduces with time elapse. Further, temperature also causes influences on the electrical resistance of the motor. All these make it very doubtful for the precision of control by detecting the rotational speed of an electric motor to immediately calculate an instantaneous torque of the motor.
Further, environmental protection makes Lithium or Lithium based rechargeable batteries a current main stream of the rechargeable batteries. The Lithium battery has a feature that is different from a Ni—Cd battery or NiMH battery in that the output voltage reduces with the reduction of the battery capacity. The conventional impact tools that are powered by Lithium batteries exhibit a significant difference in output torque between the condition when it is just fully charged and the condition when the Lithium battery has been consumed for quite a period. This makes a trouble to the general users.
In view of the above discussed problems that the conventional impact tool is not effective in carrying out precise control, the present invention is aimed to provide an impact tool that overcomes the problems.